Measurements of Mu+NO termolecular kinetics up to 520 bar: isotope effects and the Troe theory

The recombination reaction Mu + NO + MMuNO + M (M = He, N-2, CH4) has been studied by the muon spin relaxation/rotation (mu SR) technique up to 520 bar at room temperature. The reaction remains in the low pressure regime throughout. The measured termolecular addition rate constant in N-2, 8.8 +/- 0.3 x 10(-33) cm(6) s(-1), is essentially the same as that found in our earlier study at pressures below 60 bar [J. J. Pan et al., J. Phys. Chem., 1995, 99, 17160]. It is somewhat smaller in He, 7.7 +/- 1.0 x 10(-33), but larger in CH4, 12.8 +/- 2.0 x 10(-33). The Mu + NO reaction is about five times slower than the corresponding H + NO reaction. The strong collision limits of the rate constants for three H-isotopes (Mu, H, D) reacting with NO have been estimated with Troe’s formalism for unimolecular dissociation in the low pressure regime, based on the ab initio potential energy surface of Guadagnini et al. [J. Chem. Phys., 1995, 102, 774]. The Troe calculations give less than satisfactory agreement with experiment with the corresponding weak collision factor, beta(c), higher than expected by a factor of similar to 2 for H + NO. The calculated kinetic isotope effect in the strong collision limit for Mu/H is weaker than the measured effect by a factor of two giving an apparent large isotope effect in this factor, beta(c)(Mu) approximate to 1/2,beta(o)(H), possibly due to mode specific collisional stabilization.